Theatres have been entrenched in First World War dramas in this centenary year. This one, by Stephen MacDonald, dates from 1982, and like Pat Barker’s acclaimed novel Regeneration – also seen on stage recently – it explores the friendship of the poets Siegfried Sassoon and Wilfred Owen, and takes place partly at the Craiglockhart War Hospital in Scotland. While Barker’s work has a wider sweep and employs an ingenious blend of history and imaginative fiction, MacDonald draws directly on Owen and Sassoon’s poetry, letters and memoirs for his more intimate two-hander. Caroline Clegg directs with a low-key intensity that occasionally feels a little stiff and stagey. But if the production is not highly charged, it does have the muffled force of a distant explosion.

Alasdair Craig as Sassoon is prickly, ramrod straight, almost aggressive in the brusque impatience of his manner. He’s been sent to Craiglockhart – an institution for soldiers suffering from what was euphemistically known as “war neurosis,” which was, in fact, shellshock: mental breakdown due to the trauma of the slaughter at the front. In 1917, Sassoon wrote his extraordinarily controversial, defiant declaration of disgust at what he saw as the war’s needless and phoney continuation. His friend, fellow poet Robert Graves, intervened to help Sassoon avoid the disgrace of a court martial, and suggested he be sent to Craiglockhart instead. There, he meets Owen, himself a fledgling poet, and in Simon Jenkins’ poignantly fresh-faced performance, stammering, boyish, self-consciously showing Sassoon’s artistic and social inferior, and hopelessly starstruck. When he goes tapping at Sassoon’s door with a pile of books for the older man to autograph, he initially gets a frosty reception. But gradually his ardour and admiration thaws Sassoon, and a warm affection grows steadily between the two men.

Clegg and designer Lara Booth conjure a variety of locations, from Craiglockhart rooms to gentlemen’s clubs, the Scottish countryside and a Flanders dugout, with a brisk economy and the simplest of props – a desk and chair, a Tommy’s iron helmet. And Ailis Ni Riain’s score for solo cello is liltingly lovely and hauntingly elegiac. There’s scarcely a hint that Sassoon was definitely, and Owen very probably, homosexual, which, though their friendship was platonic, feels like a curious omission. And Craig is a little over-deliberate. He could show us more of the suppressed inner suffering behind Sassoon’s terseness. As it is, he comes across as faintly self-regarding, even posturing. But Jenkins is touching. Propelled by a sense of honour and a burning desire to test the limits of his own capabilities and endurance, he propels himself back into the mud and horror of the trenches, despite the opportunity of less dangerous wartime employment. It’s a determination still more moving when you remember that Owen was to lose his life just a week before the war ended, aged only 25. Among 2014’s salutes to all those who fought the Great War, this production may not win the medals, but it is quietly affecting.